Missing conversion from StringLiteral to unicode_type

[stuartarchibald]

Reporting a bug

• I have tried using the latest released version of Numba (most recent is
  visible in the change log (https://github.com/numba/numba/blob/master/CHANGE_LOG).
• I have included a self contained code sample to reproduce the problem.
  i.e. it's possible to run as 'python bug.py'.

This:

from numba import typeof, types
from numba.core.typing import Context

x = types.literal('string literal')
print(x, type(x))
print("conversion:", x.can_convert_to(Context(), types.unicode_type))

does this:

Literal[str](string literal) <class 'numba.core.types.misc.StringLiteral'>
conversion: None

I think it should be possible to convert a StringLiteral to a unicode_type but not the converse.

A can_convert_to() method needs adding to this class:

numba/numba/core/types/misc.py

Lines 40 to 41 in 3461bcb

	class StringLiteral(Literal, Dummy):
	pass

xref: https://numba.discourse.group/t/how-to-pass-string-argument-to-numba-njit-python-function-from-inside-another-whilst-specifying-signature-type/663 which hits this bug.

[gmarkall]

Would fixing this also fix #6897?